Assessment of Dissecting Aortic Aneurysm
The pathology usually involves some form of degeneration of the elastic fibres of the medial layers of the aorta. The condition often starts with a sudden tear of the intima of either the ascending or the descending thoracic aorta, often precipitated by some form of excessive hemodynamic force such as a significant elevation in blood pressure. The blood under pressure in the lumen finds its way through a cleavage in the medial layers of the aorta. The process may progress at a variable rate, resulting eventually in a double lumen aorta made by the plane of dissection in the media causing a false lumen in addition to the true lumen. The difference in the blood pressures results from the impingement of the false lumen on the true lumen, causing obstruction and the consequent lowering of the blood pressure beyond the obstruction. Depending on the location and the degree ofthese obstructions to the true lumen, one may find significant differences in the pulses as well as the blood pressures between the two arms, between the arms and the legs, as well as between the two legs. The blood pressure differences noted may also be changing with time since dissection may further progress resulting in either more complete occlusion of the true lumen, or sometimes a distal tear of exit may reopen the previously occluded vessel. Such dynamic changes are unusual in other causes of obstruction such as chronic atherosclerotic vascular disease.
This condition is most commonly seen in the lower extremities and much less frequently in the subclavian and the upper extremity vessels. Some of the subclavian stenoses may in fact be congenital rather than a result of acquired atherosclerotic process. The blood pressure in the arm on the side of the vascular obstruction will be lower, as expected. Normally, the lower extremity blood pressure is 10-15 mmHg higher than both the central aortic and the arm pressures. There are two reasons for this increase in pressure in the lower extremities: their thicker and muscular walls and the effect of reflection of the pressure pulses. The reasons for this increase in pressure in the lower extremities are discussed in Chapter 2 .
In the lower extremity, if the blood pressure is found to be equal to or lower than in the arm, then obstructive arterial disease must be considered. The obstruction may be at the level of the aorta, the iliac, or the femoral. Blood pressure measurement in the lower extremity can be carried out by using a larger cuff (8 in wide) over the thigh and auscultating over the popliteal artery. This is best done with the patient lying prone. This method probably is more accurate but may be difficult in very obese patients. To obtain a reasonably accurate systolic blood pressure in the leg, a regular cuff can be employed above the ankle and the posterior tibial or the dorsalis pedis pulses can be palpated. When the pedal pulses are not easily felt, an ultrasound Doppler probe can be used to detect the flow and assess the peak systolic pressure. When comparisons are made, one should compare the peak pressures obtained by the same technique between the upper and the lower extremities. If one uses the Doppler technique at the ankle, then one must obtain Doppler-detected radial artery flow at the wrist with the cuff over the arm for assessment of the peak pressures in the forearm.
Because coarctation of the aorta is a congenital condition, when blood pressure discrepancies are noted between the arm and the leg in children or young adults where atherosclerosis is not an issue, the clinician should be alerted to the possibility of the presence of this condition. If the coarctation is in the aortic arch before the take-off of the left subclavian artery, the pressure in the left arm will be significantly lower than the pressure in the right arm. Most coarctations usually occur after the take-off of the left subclavian, and the upper extremity pressures will be equal. Since coarctation also causes hypertension, in younger patients with hypertension, as detected in the arm blood pressures, lower blood pressures in the legs must suggest a diagnosis of coarctation. The amount of decrease in the blood pressures in the lower extremities will depend on the severity of the coarctation. When the coarctation is severe, the femoral pulses may not only be delayed but also poorly felt or not palpable.
Coarctation causes hypertension as a direct result of the obstruction limiting the size of capacitance of the aorta as well as directly raising the resistance. In addition, the decreased blood pressure distal to the coarctation will lead to stimulation of the juxtaglomerular cells to produce more renin The latter will increase the angiotensin II, which is a potent vasoconstrictor and will result in hypertension.
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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...